Auroral Dynamics from Large to Small Scales

Abstract:

One of the still unsolved problems of auroral physics, besides the details of the acceleration process itself, is the relationship between the auroral acceleration region and the background plasma. The dynamics of large quasi-stable arcs as well as the fast motion of small-scale structures have to be related to the interaction between the accelerating quasi-static potential or electromagnetic waves with the background plasma. We will demonstrate ground-based and space-based observations of auroral forms together with ionospheric radar or spacecraft measurements to discuss three main motions and scales of auroral forms. Quiet evening arcs represent the largest structures and move with the slowest speed equatorward. The substorm bulge with its fast motion poleward represents the medium-size class of auroral scales. Finally, rays and curls represent the smallest scales with the fastest motions along the arcs in mostly east-west direction. The discussion of these different scales and their relation to field-aligned currents and distortions in the auroral acceleration region will also be key to understanding the appearance of new arcs, their deformation and splitting, and their relative motion with respect to the background plasma.